You've probably seen this optical illusion before, in which two lines of identical length appear to be different by adding opposite-facing arrows to them. It's a very powerful effect - so much so that it overwhelms all other visual information.

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That's the finding of researchers at the University of London, who in an experiment asked participants to search for a vertical line among a bunch of other, slightly tilted lines. All the lines had arrows on their ends, some pointing in and some pointing out, and all just different enough to make the lines appear to be of different lengths thanks to this illusion.

The participants' eyes were consistently drawn to the line that appeared longest, even if it clearly wasn't the vertical line. Chief researcher Dr. Michael Proulx argues that this is indicative of how our brain reflexively calculates size and length before anything else, and this process happens fast enough to guide where the eyes look, even if we consciously know that we need to be looking elsewhere.

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Dr. Proulx explains:

"The surprising difference here is that the perceived longer line not only captured their attention, but was even more distracting than the sudden appearance of something new as shown in prior research. This suggests that many visual illusions might be so effective because they tap into how the human brain reflexively processes information. If an illusion can capture attention in this way, then this suggests that the brain processes these visual clues rapidly and unconsciously. This also suggests that perhaps optical illusions represent what our brains like to see."

This particular illusion is known as the Müller-Lyer illusion, named for the man who first discovered it, F.C. Müller-Lyer in 1889. The researchers hope to take this further by examining how strong this visual reflex is in those whose perception has been otherwise compromised, such as those with conditions like autism and schizophrenia. Dr. Proulx comments:

"A number of conditions exhibit differences in attention, such as Autism and schizophrenia, and it would be useful to see whether visual illusions are still given priority even when other aspects of attention are affected."